A fuel cell stack is a power generation system which oxidizes a fuel through an electrochemical process to thereby directly convert energy released as a result of the oxidation reaction into electric energy. The fuel cell stack has a membrane-electrode assembly in which a polymer electrolyte membrane, which selectively transports hydrogen ions, is sandwiched by a pair of electrodes made of porous materials. Each of the pair of electrodes includes: a catalyst layer that contains, as a main ingredient, carbon powder supporting a platinum-based metal catalyst and contacts with the polymer electrolyte membrane; and a gas diffusion layer formed on a surface of the catalyst layer, the gas diffusion layer having both air permeability and electronic conductivity.
In fuel cell systems of this type, if a cell voltage which has been set based on system-requested power becomes equal to or higher than a predetermined voltage and platinum in the catalyst layer is exposed to a high potential of equal to or higher than a predetermined value, dissolution (ionization) of the platinum might occur and cause degradation of output characteristics. In addition, if the fuel cell continues to be operated within an operation zone where the cell voltage becomes an oxidization voltage, an oxide film might be formed on a surface of the platinum catalyst in the catalyst layer and cause degradation of the output characteristics.
Patent Document 1 discloses a technique for using the oxide film formed on the surface of the platinum catalyst as a protective film for suppressing the dissolution of platinum. Specifically, if a target value of the cell voltage which has been set based on the system-requested power is equal to or higher than a predetermined film dissolution-starting voltage at which the platinum starts dissolving, the cell voltage is held at a predetermined oxide film formation voltage for a predetermined time period in order to form an oxide film on the surface of the platinum catalyst and then the cell voltage is set to the target value.